Authorities are investigating bomb threats sent electronically to businesses, universities and government buildings in more than a dozen cities around the country on Thursday, prompting multiple evacuations.

“We are aware of recent bomb threats made in cities around the country & remain in touch w/ our law enforcement partners to provide assistance,” the Bureau of Alcohol, Tobacco, Firearms and Explosives tweeted Thursday afternoon. “As always, we encourage the public to remain vigilant & to report suspicious activities that could represent a threat to public safety.”

Bomb threats were reported in more than a dozen places, including New York City, Chicago, Detroit, Atlanta, Oklahoma City and San Francisco. Several universities — including the University of Washington in Seattle and Penn State University in State College, Penn., — also received threats.

There have been no detonations, and authorities have said the threats do not appear to be credible.

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It is not yet clear if the threats are connected. But many of them came in emails from scammers demanding a bitcoin payment. In Cedar Rapids, Iowa, police said the threats came in a “robo-email saying there is a bomb threat to their business unless they pay money in Bitcoins.”

The department, which said there was no evidence the threats were credible, posted the text of one of the emails, which carried the subject line “Do not waste your time.”

“My man hid an explosive device (Hexogen) in the building where your business is conducted,” the email said. “I can call off my man if you make a transfer. 20.000 dollars is the cost for your life and business. Pay it to me in BTC and I warrant that I will withdraw my man and the device won’t detonate.”

Other emailed threats had similar wording and grammar, with slight differences in the text.

While no physical bombs have been reported by police, the threats did impact businesses and institutions in some areas. Aurora, Illinois’s city hall, for example, was evacuated this afternoon, according to the Chicago Tribune. Police gave the all-clear for employees to return to the building after approximately 45 minutes.

The Raleigh, North Carolina office building housing the News & Observer was also evacuated Thursday afternoon after the newspaper received a bomb threat via email.

Though the threat “did not appear to be legitimate,” according to the paper’s publisher, Sara Glines, the company’s policy is to “evacuate our staff when there is a threat of any kind,” the newspaper itself reported.

Colorado’s Columbine High School also reported receiving a threat on Thursday, though it was not immediately confirmed to be related to the string of bomb threats across the country.

An individual had called to claim that they had placed explosive devices inside the school, and that they were hiding outside the school with a gun, the AP reported. Columbine was infamously attacked in 1999 when two disgruntled students shot and killed 12 classmates and one teacher.

No credible threat was found, according to Jefferson County Sheriff’s Office spokesman, Mike Taplin, per AP.